As for a capacitor having high capacitance and low ESR (equivalent series resistance) used in various electronic devices, an aluminum solid electrolytic capacitor and a tantalum solid electrolytic capacitor are known.
The solid electrolytic capacitor is produced by sealing a solid electrolytic capacitor element in which an aluminum foil having fine pores in the surface layer or a tantalum powder sintered body having fine pores in the inside is used as one electrode (electric conductor) and which comprises a dielectric layer formed on the surface layer of the electrode, the other electrode (usually a semiconductor layer) provided on the dielectric layer, and an electrode layer stacked on the other electrode. In comparison among capacitors using electric conductors having the same volume, the smaller the size of the fine pores of the conductor and the larger the number of the pores, the larger the surface area of the conductor inside and the larger the capacitance of the capacitor produced from the electric conductor can be.
The dielectric layer is formed by an electrochemical method called chemical formation. An example of the forming process is a method where an electrically conducting layer is dipped in an electrolytic solution containing a mineral acid (e.g., phosphoric acid, sulfuric acid) or a salt thereof, or an organic acid (e.g., acetic acid, adipic acid, benzoic acid) or a salt thereof dissolved therein and a predetermined voltage is applied between the electric conductor serving as an anode and a cathode separately provided in the electrolytic solution. A part of the electrolyte used for the chemical formation is incorporated into the dielectric layer.
JP-A-S50-100570 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”; patent Document 1, related application; U.S. Pat. No. 3,864,219) describes chemical formation in an electrolytic solution using quaternary ammonium salt. Also, JP-A-S50-102861 (patent Document 2) describes chemical formation in an electrolytic solution using boric acid or the like.
As for the semiconductor layer, an organic or inorganic compound is used but in the light of heat resistance or low ESR property of the produced capacitor, an electrically conducting polymer is used in preference. The electrically conducting polymer is a polymer having a high electric conductivity of 10−2 to 103 S·cm−1. The high electric conductivity is prepared by adding an electron-donating compound called a dopant to a polymer having a planer conjugated double bond (generally, insulating material or a polymer having a very low electric conductivity). Specific examples of the method for forming an electrically conducting polymer as the semiconductor layer include a method of polymerizing a monomer capable of being polymerized to an electrically conducting polymer in the fine pores of the electric conductor by supplying thereto an appropriate oxidizing agent or an electron in the presence of a dopant. The dopant is entrained upon polymerization of the monomer and strong interaction with the polymer having a conjugated double bond occurs, whereby an electrically conducting polymer is obtained.
A solid electrolytic capacitor is required to have a high reliability. One example of examining the reliability by acceleration is a high heat load test. In the test, for example, solid electrolytic capacitors are left standing at 105° C. for thousands of hours while applying a rated voltage of the capacitor and those not whose electric properties have not degraded are determined as acceptable.    [Patent Document 1]    Japanese Patent Application Laid-Open No. S50-100570    [Patent Document 2]    Japanese Patent Application Laid-Open No. S50-102861